The present invention pertains to the electrical communication art and, more particularly to apparatus for recovering the clock signal in a phase shift keyed encoded data signal.
In numerous digital communication systems, the information data is processed with a stable clock frequency signal prior to data transmission. An example of such a system is found in the mobile telephone communication art. There, addressing or "paging" data signals at a base station originate in non-return-to-zero form. Such signals are not suited for transmission over communication channels. As such, they are commonly combined with a stable frequency clock signal to produce a phase shift keyed transmission signal. In fact, the common form of such transmission signals is Manchester encoded data.
For the system to operate properly, data transmissions from the base station must be received, and processed at the mobile station with efficiency and precision. To accurately reproduce the original non-return-to-zero information from the received Manchester signal, apparatus must be provided for the recovery of the clock signal. It is imperative that a means be provided to identify the correct clock phase and then to prevent the mobile station from locking on to the opposite phase during noise and periods of no channel conditions which occur due to multipath fading and other obstructions.
In one prior art approach to extracting the clock from Manchester encoded input data signals, the input signal is full wave rectified and then bandpassed, at the clock frequency, to recover bit clock information. The bit clock information drives a phase locked loop having a voltage controlled oscillator, the output from which is used as the recovery clock signal.
This prior art system exhibits numerous shortcomings. For example, to avoid spurious responses closely matched and, thus, expensive rectifier diodes must be used. Further, the system is subject to clock recovery drift as a function of the number of transitions in the input data. That is, as the input data tends to approach logic one or logic zero symbols, this prior system exhibits a clock drift which leads to recovery errors.
Thus, there has been a long felt need in the data communication art for a precision, high speed clock recovery system.